A central feature of allergic reactions is the aggregation of the high-affinity IgE Receptor, (also known as FCεR1), to initiate a change in the behavior of the cell expressing the receptor. The high affinity Fc epsilon receptor is associated with the activation of mast cells and basophils and the triggering of allergic reactions and anaphylactic shock. Moreover, knockout mice for the FCεR1 alpha chain (FCεR1α) are unable to mount IgE-mediated anaphylaxis (Dombrowicz et al., 1993, Cell 75, 969-976).
FCεR1 is not only expressed on so-called effector cells of allergic reaction, i.e. mast cells and basophils, but there is clear evidence that Langerhans cells, monocytes (or a subset of monocytes), and cells derived from monocytes, such as dendritic cells, also express this receptor (e.g., Wang, B., et al., 1992 J. Exp. Med. 175, 1353-1365, and Maurer, D., S. et al., 1996, J. Immunol. 157, 607-616). Moreover, expression levels of FCεR1 on these cell types correlates with atopic status (Maurer, D. S, et al., 1994, J. Exp. Med. 179, 745-750.
The high affinity IgE binding receptor expressed on mast cells and basophils is a tetrameric receptor comprised of the IgE-binding α chain, the membrane-tetra-spanning β chain, and a disulfide-linked homodimer of the γ chains. FCεR1 expressed on other cells of the immune system e.g., dendritic cells is a trimeric receptor comprised of a single αchain and the γ chain homodimer. The alpha chain of FCεR1 binds IgE molecules with high affinity (K D of about 10-9 to 10-10 moles/liter (M)) at a stoichiometry of 1:1, whereas the β and γ chains of FCεR1α are signal transduction modules. Loss of FCεR1α chain of this receptor prevents surface expression of the receptor complex and ablates IgE binding.
Crosslinking of IgE molecules bound to FCεR1 by an allergen activates a signaling cascade in mast, basophil and other immune cells that results in the release of chemical mediators responsible for numerous adverse events. That is, activation of the FCεR1 signaling cascade leads to the immediate (i.e., within 1-3 min. of receptor activation) release of preformed mediators of atopic and/or Type I hypersensitivity reactions (e.g., histamine, proteases such as tryptase, etc.) via the degranulation process. Such atopic or Type I hypersensitivity reactions include, but are not limited to, anaphylactic reactions to environmental and other allergens (e.g., pollens), hay fever, allergic conjunctivitis, allergic rhinitis, allergic asthma, atopic dermatitis, eczema, urticaria. It also leads to the synthesis and release of other mediators, including leukotrienes, prostaglandins and platelet-activating factors (PAFs), that play important roles in inflammatory reactions. The resulting smooth muscle contraction and acute bronchoconstriction constitutes the so-called early phase response of an asthmatic reaction. Additional mediators that are synthesized and released upon crosslinking FCεR1 receptors include cytokines and chemokines such as IL-4, IL-5, IL-6, TNF-α, IL-13 and MIP1-α) and nitric oxide. These mediators contribute to delayed bronchoconstriction and an increase in airway hyperreactivity the so-called late phase response in an asthmatic reaction. Pathologically, the late-phase response is characterized by cellular inflammation of the airway, increased bronchovascular permeability, and increased mucus secretion. Accordingly, in mice lacking the FCεR1α chain, airways hyper-responsiveness is ablated and inflammation reduced following aeroallergen sensitization and challenge (Taube, C et al., 2004, J. Immunol 172, 6398-6406).
Furthermore, this cytokine and chemokine mileu further enhances allergic sensitization by promoting naïve T cell differentiation into Th2 cells via IL4-R signaling. IL4 also promotes class-switching to IgE in B cells and there is mounting evidence that in addition to Th2 cells, basophils are a primary source of IL-4 and IL-13 (see e.g., Gauchat, J. F. et al., 1993, Nature, 365, 340-343). Basophils secrete quantities similar to T cells, and because the frequency of basophils (with their panspecific IgE) is much greater than that of antigen-specific Th2 cells, potentially basophils may generate most of the IL-4 present in tissues via allergen-activated FCεR1 (see e.g., Devouassoux, G. et al., 1999, J. Allergy Clin. Immunol. 104, 811-819). Hence, significant reductions in IL-4 and IL-13 secretion from basophils could have long-term consequences for the synthesis of IgE.
An important advancement in the treatment of allergic rhinitis and asthma in recent years has been the development of monoclonal antibodies specifically targeting IgE. Studies in allergic rhinitics with the anti-IgE humanised monoclonal antibody (omalizumab, Xolair™) have demonstrated significant reduction in nasal symptoms and improvement in quality of life (Owen, C., 2007, Pharmacology and Therapeutics 113, 121-133). More recently it has been demonstrated that omalizumab is particularly effective in patients with both asthma and rhinitis (Vignola et al., 2004, Allergy 59, 709-717) and also in poorly controlled asthmatics despite high-dose inhaled corticosteroid therapy and particularly those at the more severe end of the spectrum. In these patients, the most significant benefit is in the reduction in exacerbation rate and airway inflammation (Nowak., 2006 Resp. Med. 100, 1907-1917). Ex vivo studies have shown that one of the mechanisms by which anti-IgE may mediate its clinical benefit is by down-regulating the expression and function FCεR1 on surface of circulating basophils, tissue-resident mast cells (Beck et al., 2004, J Allergy Clin Immunol. 114, 527-30) and dendritic cells. There remains, however, a need for other therapeutics to treat disease associated with aggregation of the high-affinity IgE Receptor.
Alteration of gene expression, specifically FCεR1α gene expression, through RNA interference (hereinafter “RNAi”) is a one approach for meeting this need. RNAi is induced by short double-stranded RNA (“dsRNA”) molecules. The short dsRNA molecules, called “short interfering RNA” or “siRNA” or “RNAi inhibitors” silence the expression of messenger RNAs (“mRNAs”) that share sequence homology to the siRNA. This can occur via cleavage of the mRNA mediated by an endonuclease complex containing a siRNA, commonly referred to as an RNA-induced silencing complex (RISC). Cleavage of the target RNA typically takes place in the middle of the region complementary to the guide sequence of the siRNA duplex (Elbashir et al., 2001, Genes Dev., 15, 188). In addition, RNA interference can also involve small RNA (e.g., micro-RNA or miRNA) mediated gene silencing, presumably though cellular mechanisms that regulate chromatin structure and thereby prevent transcription of target gene sequences (see for example Allshire, 2002, Science, 297, 1818-1819; Volpe et al., 2002, Science, 297, 1833-1837; Jenuwein, 2002, Science, 297, 2215-2218; and Hall et al., 2002, Science, 297, 2232-2237).